Expected to contain a CODE reference which returns either true or false.

If the code returns true, the action should be enabled/shown, otherwise it shouldn't, usually because it won't make sense to use this action without whatever_is_checked_by_the_code. (For example, UNDO can't be used if there was no change which could be undone.)

The CODE receives a list of objects which should help with the decision:

config Contains the current configuration object
editor The current editor object
document The current document object
main The main Wx object

Use this with caution! As this function is called very often there are few to no checks and if this isn't a CODE reference, Padre may crash at all or get very slow if your CODE is inefficient and requires a lot of processing time.